This invention relates generally to the field of air moving apparatus such as fans and blowers. More specifically, the invention relates to an impeller for use in fans of the transverse type. Transverse fans are also known as cross-flow or tangential fans.
The operating characteristics and physical configuration of transverse fans make them particularly suitable for use in a variety of air moving applications. Their use is widespread in air conditioning and ventilation apparatus. Because such apparatus almost always operates in or near occupied areas, a significant design and manufacturing objective is quiet operation.
FIG. 1 shows schematically the general arrangement and air flow path in a typical transverse fan installation. FIG. 2 shows the main features of a typical transverse fan impeller. Fan assembly 10 comprises enclosure 11 in which is located impeller 30. Impeller 30 is generally cylindrical and has a plurality of blades 31 disposed axially along its outer surface. Impeller 30 comprises several modules 32, each defined by an adjacent pair of partition disks 34 or by one end disk 33 and one partition disk 34. Between each adjacent pair of disks longitudinally extend a plurality of blades 31. Each blade is attached at one of its longitudinal ends to one disk and at the other end to the other disk of the pair. A given impeller may comprise multiple modules as depicted in FIG. 2 or but a single module, where the blades attach at either end to an end disk. The choice of a single or multiple module configuration depends upon such factors as fan size, construction material strength and weight and the like. As impeller 30 rotates, it causes air to flow from enclosure inlet 21 through inlet plenum 22, through impeller 30, through outlet plenum 23 and out via enclosure outlet 24. Rear or guide wall 15 and vortex wall 14 each form parts of both inlet and outlet plena 22 and 23. The general principles of operation of a transverse fan need not be elaborated upon except as necessary to an understanding of the present invention.
When a transverse fan is operating, it generates a certain amount of noise. One significant component of the total noise output of the fan is a tone having a frequency related to the rotational speed of the fan multiplied by the number of fan blades (the blade rate tone). The passage of the blades past the vortex wall produces this blade rate tone. Discrete frequency noise is in general more irritating to a listener than broad band noise of the same intensity. The blade rate tone produced by the typical prior art transverse fan has limited the use of such fans in applications where quiet operation is required.
At least one prior art disclosure has proposed a means of reducing the blade rate tonal noise produced by a transverse fan. U.S. Pat. No. 4,538,963 (issued 3 Sep. 1985 to Sugio et al.) discloses a transverse fan impeller in which the circumferential blade spacing (called pitch angle in the patent) is random.
Another patent, U.S. Pat. No. 5,266,007 (issued 30 Nov. 1993 to Bushnell et al.), one inventor of which is also an inventor of the present invention and the assignee of which is the same as the assignee of the present invention, discloses a transverse fan impeller that is effective in reducing the blade rate tonal noise in a transverse impeller by varying the angular spacing of the impeller blades in a nonuniform but also nonrandom manner.
It is the interaction between air flow, rather than the fan blades themselves, and the vortex wall that produces the blade rate tone in a transverse fan. Therefore one can reduce the blade rate tone by any means that reduces the regularity of the air flow interaction at the vortex wall.